The first two are very well established. Once you're above about 2,500 meters (8,000 feet), your risk of AMS increases the higher you go; and you should avoid ascending more than about 300 meters (1,000 feet) per day. But what about previous history? It's one of those risk factors that seems obvious, but may not actually be supported by reliable evidence. That's what Martin MacInnis and his colleagues at the University of British Columbia's Environmental Physiology Lab set out to investigate in a meta-analysis just published in the British Journal of Sports Medicine. They aggregated data from 17 studies with nearly 8,000 subjects, and found... well, not much. Previous history turns out not to be a great predictor of future AMS, but that may be in part because the data itself wasn't that great -- for example, if you don't know the peak height and rate of ascent of a previous climb, then it's very difficult to compare it to a future climb.

In quantitative form, what they found is that history as a diagnostic predictor had a sensitivity of 0.50 (i.e. the true positive rate: the number of subjects with past history of AMS who subsequently developed AMS again, divided by the total number of subjects with past history of AMS). That's not very useful.

The specificity was 0.72 (i.e. the true negative rate: the number of people with no past history of AMS who once again didn't develop AMS, divided by the total number of people with no past history of AMS). This is a bit more useful: it suggests that if you've been to altitude before with no problems, you have a reasonable chance of doing it again.

In the end, the strongest conclusion from the meta-analysis is that better data is needed if previous history is to be kept as a predictor of AMS risk. In particular, studies need to make sure the details of previous climbs are known so that you're comparing apples to apples. There are a number of other data problems the authors point out -- for example, people who've previously had AMS are more likely to use prophylactic drugs like Diamox in subsequent climbs, which may be one of the reasons for the low sensitivity in this analysis. For now, the message is don't blame your genes or your fate for AMS; instead, control the factors you can control, like rate of ascent!